RSpec: expect exact exception to avoid RSpec warnings

[olleolleolle]

This PR makes the test suite run without outputting the warning:

.WARNING: Using the `raise_error` matcher without providing a specific error or 
message risks false positives, since `raise_error` will match when Ruby raises a 
`NoMethodError`, `NameError` or `ArgumentError`, potentially allowing the 
expectation to pass without even executing the method you are intending to call. 
Actual error raised was #<RuntimeError: please install postgresql>. Instead consider 
providing a specific error class or message. This message can be suppressed by setting: 
`RSpec::Expectations.configuration.on_potential_false_positives = :nothing`. Called from 
/opt/src/opensource/ruby/pgtester/spec/pg_tester_spec.rb:36:in `block (4 levels) in <top 
(required)>'.